The present invention relates generally to the field of scanning electron microscopes used as test probes for visualizing and testing of integrated circuits and more particularly to an improved magnetic lens system for focusing the electron beam present in such test probes onto the surface of the integrated circuit and for collecting the secondary electrons produced by the interaction of the electron beam and the integrated circuit.
As a result of the progress in the design and fabrication of integrated circuits, it has become possible to create circuits having millions of conductors and transistors in which the individual conductors are of the order of one to two microns These circuits are too small and complex to be amenable to testing and analysis by techniques using mechanical probes. The mechanical probes tend to capacitively load the circuits under test, thus altering the behavior one wishes to measure. Further, the mechanical probes may actually physically damage the minute conductors with which they come in contact. Finally, the number of conductors which must be examined to debug a VLSI integrated circuit is rapidly becoming too large to be amenable to manual measurement one node at a time. As a result, test probes based on electron beams have been developed. These test probes provide a means for measuring the potential on minute conductors as well as a means for forming an image of the conductors and the surrounding circuitry without any physical damage thereto.
Such an electron beam test probe is described in the above cited copending application which is hereby incorporated by reference. In general, electron beam test probe systems measure the potential at a specified point on the surface of the integrated circuit by sensing the energy distribution of the secondary electrons produced when the point in question is bombarded by electrons. The electron beam test probe system includes a means for generating an electron beam which may be directed at any point within a specified region, referred to as the field of view, on the integrated circuit surface. The interaction of this electron beam with the surface of the integrated circuit results in the emission from the surface of secondary electrons whose energy distribution is related to the potential on the surface of the integrated circuit at the point where the electron beam is currently focused. The potential on the surface may be deduced from the fraction of the energy distribution which exceeds a predetermined value. This fraction is determined by detecting the number of secondary electrons which have sufficient energy to overcome a potential barrier and reach an electron detector.
The size of the field of view of prior art electron beam test probe systems is limited by aberrations in the electron beam optical system which increase with distance from the axis of the final lens which focuses the electron beam to a spot on the integrated circuit surface. These aberrations are significantly worse than those encountered in scanning electron microscopes, since the specimen must be spaced a considerable distance from the final lens in order to accommodate secondary electron collection hardware and hardware needed to couple signals to the integrated circuit being tested. As a result, a long focal length magnetic lens must be used. The aberrations in question increase with the focal length of the magnetic lens. In addition, as explained in the above mentioned copending application, it is desireable to use a low energy electron beam to minimize circuit damage. The extent to which chromatic aberration limits the performance of the electron beam test prob system depends on the electron beam energy and spread in electron energies within the electron beam. The performance of the system depends on the ratio of the spread in energies to the beam energy. Hence, chromatic aberration affects low energy electron beam test probe systems to a greater extent that it does high energy systems. As a result of these factors, prior art electron beam test probe systems are not able to maintain high spatial resolution and still have a field of view which encompasses the entire integrated circuit being examined.
The above cited copending application describes an improved electron beam test probe system in which a collimating magnetic lens is used both to focus the electron beam to a point on the integrated circuit surface and to collect the secondary electrons produced in response to the electron beam bombardment of said integrated circuit. This magnetic lens significantly reduces the distance needed to house secondary electron collection hardware. As a result, a shorter focal length magnetic lens with improved chromatic aberration is obtained. Although the use of this magnetic lens significantly decreases chromatic aberrations, it is still not sufficiently aberration free to allow a field of view which encompasses an entire integrated circuit. In addition, the field of view of the improved system is limited by the collection efficiency of the magnetic lens The collection efficiency of the magnetic lens decreases with distance from the axis of the magnetic lens. As explained in the above mentioned copending application, it is important to maintain a high collection efficiency to prevent artifacts in the measured potentials.
Broadly, it is an object of the present invention to provide an improved collimating magnetic lens having a field of view sufficiently large to encompass an entire integrated circuit.
It is a further object of the present invention to provide a collimating magnetic lens with decreased aberrations at points far from the axis of said lens.
It is a still further object of the present invention to provide a collimating magnetic lens which has high collection efficiency at points far from the axis of said lens.
These and other objects of the present invention will become apparent from the following detailed description of the invention and the accompanying drawings.